CARDIID. 195 
The transitions between the type and the following subgenera 
are so gradual that the latter must be regarded as of little value. 
mMonopacna, Hichw., 1838. Hinge with a single tooth. A. 
Caspicum, Hichw. (exvi, 85). 
DIDACNA, Hichw., 1838. Hinge with two teeth. A. Donaci- 
formis, Schriter (exvi, 85). 
PROSODACNA, Tournouer, 1882. A. macrodon, Desh. Tertiary ; 
Crimea. 
Hemicarpium, Cuvier, 1817. 
Distr.—28 sp. Tropical. H. cardissa, Linn. (cxvi, 87). 
Hi, hemicardium, Linn. (exvi, 88). 
Shell cordiform, the posterior slope strongly depressed and 
bordered by a carina; lunule simple; cardinal teeth distinct, 
more or less twisted ; surface radiately ribbed. 
FRAGUM, Bolten, 1798. Anterior side short and truncated ; 
ribs tuberculated. H. unedo, Linn. (cxvi, 89). 
cTENOCARDIA, H. and A. Adams, 1855. Like Fragum, but ribs 
spinose. H. hystrix, Brod. (exvi, 90, 91). 
LUNULICARDIA, Gray, 1847. Lunule depressed, surrounded by 
a deep broad channel; ribs nearly smooth and flattened on the 
posterior slope. H. retusa, Linn. (cxvi, 92). 
ConocarpiuM, Bronn. 
Syn.—Lychas, Stein. Pleurorhynchus, Ph. Lunulocardium) 
Munster. Arcites, Martin. 
Distr.—Fossil, 30 sp. U. Silurian — Carb.; North America, 
Europe. C. aliforme, Sowb. (exvi, 93). C. Hibernicum, Sowb. 
(exvii, 100). 
Shell equivalve-rigonal, conical and gaping in front, truncated 
behind, with a long siphonal tube near the umbones; anterior 
slope radiately, posterior obliquely striated; margins strongly 
crenulated within; hinge with anterior and posterior laminar 
teeth ; ligament external. 
The truncated end has usually been considered anterior, a 
conclusion which seems incompatible with the vertical position 
and burrowing habits of most free and equivalve shells; if com- 
pared with Adacna the large gape will be for the foot, and the 
long tube siphonal. C. Hibernicum has an expanded keel, like 
Hemicardiuminversum. The shell-structure is prismatic-cellular, 
as first pointed out by Sowerby; but the cells are cubical, and 
much larger than in any of the Aviculade. In Cardium, the 
outer layer is only corrugated or obscurely prismatic-cellular. 
LITHOCARDIUM, Desh. Shell triangular, keeled; anterior side 
very short; hinge-teeth 1:2, directed backwards; posterior lat- 
erals 2:1; anterior muscular pit minute, posterior impression 
large, remote from the hinge. ZL. cymbulare, Lam., exhibits 
slight indications of a byssal sinus in the front margins of the 
